RE: [lwip-users] TCP Checksum errors.

[Nick Thomas]

> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden Behalf
> Of address@hidden
> Sent: 19 September 2008 20:07
> To: Mailing list for lwIP users
> Subject: Re: [lwip-users] TCP Checksum errors.
>
>
> Nick Thomas wrote:
> > But, I still have a real problem, in that the HTTP response doesn't come
> > back to the PC based browser.
> > It is these responses that wireshark is moaning about, surely not a
> > coincidence?
> >
> Wireshark may report wrong checksums in packets sent by the PC on which
> wireshark runs - but only by packets sent using the windows internal
> TCPIP stack. So if wireshark reports checksum errors in responses sent
> by the http server on your embedded device back to the PC, these are
> most certainly real checksum errors: I don't know a driver that
> intentionally inserts a wrong checksum into a received frame while
> checking it...
>
> To be sure: if the PC re-sents the same packet again, it is
> retransmitting, which means it didn't get the response and the wireshark
> was not false positive.
>
> Simon
>

Yes, wireshark (running on my PC) is complaining about checksums in packets
sent from my non-PC based development unit.
The PC then re-sends it's initial HTTP GET, and again receives a packet with
a bad checksum.
So, this doesn't appear to be a PC problem.

I have a similar codebase running on a different hardware platform which
doesn't exhibit this problem. However, the ethernet chips involved are
different, and the LWIP stack has some hardware related registers setup
differently. This is looking more like a problem with this specific unit,
and it's LWIP port :(

So, when wireshark suggests that it could be a 'TCP Checksum offload'
problem, is it referring to a setting in the development unit ethernet chip?


Thanks for the advice so far.


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